Responses of phytoplankton and epilithon during acidification and early recovery of a lake

1. Lake 302S in the Experimental Lakes Area of Canada was acidified from pH 6.7 (1981) to 5.1 (1986). The pH was further reduced to 4.5 in 1987 and held at that level until 1991. From 1992 to 1995, the pH was allowed to increase to a target value of 5.8.
2. The response of the phytoplankton community to decreasing pH from 6.0 to 5.1 was similar to that observed in another experimentally acidified lake (223) and in other atmospherically acidified lakes. Acidification affected species diversity of both the phytoplankton and epilithon. Phytoplankton diversity was positively correlated with pH. Epilithic algal diversity was more variable and did not correlate with pH.
3. Phytoplankton biomass was enhanced by acidification as the assemblage shifted from a dominance of chrysophytes to large dinoflagellates ( Gymnodinium sp. and Peridinium inconspicuum ). Epilithon biomass was unaffected, but dominance shifted from filamentous cyanophytes ( Lyngbya ) to acidophilic diatoms ( Tabellaria quadriseptata and Anomoeonis brachysira ).
4. The only taxon to be similarly affected in both the phytoplankton and epilithon was the cyanobacteria, being significantly reduced below pH 5.1. During early recovery (pH 5.5–5.8), cyanobacteria increased and species present prior to acidification recolonized both habitats.
5. In the early stages of recovery, planktonic and benthic assemblages remained more similar to acidified than natural assemblages, but more profound change began at pH > 5.5.     

Composition of inorganic and organic nutrient sources influences phytoplankton community structure in the New River Estuary, North Carolina

The New River Estuary, NC, is a nutrient-sensitive, eutrophic water body that is prone to harmful algal blooms. High annual loading from the watershed of varying nutrient forms, including inorganic phosphorus and inorganic and organic nitrogen, may be linked to the persistence of algal blooms in the estuary. In order to evaluate phytoplankton response to nutrient inputs, a series of in situ nutrient addition experiments were carried out during June 2010 to July 2011 on water from an estuarine site known to support algal blooms. Estuarine water was enriched with nutrients consisting of individual and combined sources of dissolved inorganic nitrogen, orthophosphate, urea, and a natural dissolved organic nitrogen (DON) addition derived from upstream New River water. The combined inorganic N and P addition most frequently stimulated phytoplankton biomass production as total chlorophyll a. The responses of diagnostic (of major algal groups) photopigments were also evaluated. Significant increases in peridinin (dinoflagellates), chlorophyll b (chlorophytes), and myxoxanthophyll (cyanobacteria) were most frequently promoted by additions containing riverine DON. Significant increases in zeaxanthin (cyanobacteria) were more frequently promoted by inorganic nitrogen additions, while increases in fucoxanthin (diatoms) and alloxanthin (cryptophytes) were not promoted consistently by any one nutrient treatment. Evaluating the impact of varying nutrient forms on phytoplankton community dynamics is necessary in order to develop strategies to avoid long-term changes in community structure and larger-scale changes in ecosystem condition.     

Comparative responses of phytoplankton during chemical recovery in atmospherically and experimentally acidified lakes1

Twenty lakes recovering from a century of atmospheric acid deposition over Northeastern Ontario were resurveyed for phytoplankton following a 20‐year period and were compared with a 23‐year study of an experimentally acidified lake, L302S (Experimental Lakes Area, ON, Canada). Phytoplankton species significantly tracked abiotic changes during both acidification and chemical recovery in all lakes based on concordance testing. However, ordination analyses showed that many phytoplankton communities had not returned to their preacidification state. Significant explanatory variables of taxonomic responses were pH, dissolved organic carbon (DOC), and inorganic nutrients (N, P), based on canonical correspondence analysis (CCA). Increases in DOC and pH influenced a significant taxonomic shift from acid‐tolerant dinoflagellates to a diverse assemblage of cyanobacteria, chlorophytes, and diatoms. Declining nitrogen levels defined a secondary environmental gradient, which was characterized by a decrease in filamentous green algal abundance. L302S remained remote in ordination space from the more chronically and heavily polluted lakes in Northeastern Ontario, indicating that experimental acidification provided a conservative estimate of the true damage to atmospherically polluted lakes. However, L302S did increasingly resemble lakes in Northeastern Ontario, suggesting that experimental acidification simulated the impacts of moderate levels of atmospheric pollution. Our findings demonstrate the importance of ecological history in understanding the responses by boreal lake ecosystems to environmental change.     

The effects of nutrient addition and pH manipulation in bag experiments on the phytoplankton of a small acidic lake in West Virginia,USA

In situ bag experiments were performed during summer and autumn in a small acidic lake, Tibbs Run Lake, West Virginia, USA. The objective was to evaluate phytoplankton responses to pH manipulation and nutrient addition. Increasing the pH from below 4.5 to over 6.3 resulted in great declines in phytoplankton biovolume. There was also a succession from dinoflagellates (Peridinium inconspicuum to small chlorophytes. The trend was more rapid where phosphorus (P) additions were made along with pH enhancement. During summer, P limitation was indicated, while nitrogen (N) appeared to limit production in autumn. In both seasons, nutrient additions greatly altered the phytoplankton composition in high pH treatments, but had no discernable effects at (the natural) low pH. A low pH, P addition treatment in autumn was the single exception. When N was subsequently added, phytoplankton composition changed dramatically, probably because the proceeding P additions caused severe secondary N-limitation. In general, however, the results supported the view that phytoplankton compositional responses to nutrient additions are suppressed in low pH, relative to high pH lake water.     

Composition and distribution of the pelagic and sympagic algal assemblages in the Laptev Sea during autumnal freeze-up

The phytoplankton and ice algal assemblages in the SiberianLaptev Sea during the autumnal freeze-up period of 1995 aredescribed. The spatial distribution of algal taxa (diatoms,dinoflagellates, chrysophytes, chlorophytes) in the newly formedice and waters at the surface and at 5 m depth differed considerablybetween regions. This was also true for algal biomass measuredby in situ fluorescence, chlorophyll (Chl) a and taxon-specificcarbon content. Highest in situ fluorescence and Chl a concentrations(ranging from 0.1 to 3.2 µg l^–1) occurred in surfacewaters with maxima in Buor Khaya Bay east of Lena Delta. Thealgal standing stock on the shelf consisted mainly of diatoms,dinoflagellates, chrysophytes and chlorophytes with a totalabundance (excluding unidentified flagellates <10 µm)in surface waters of 351–33 660 cells l^–1. Highestalgal abundance occurred close to the Lena Delta. Phytoplanktonbiomass (phytoplankton carbon; PPC) ranged from 0.1 to 5.3 µgC l^–1 in surface waters and from 0.3 to 2.1 µg Cl^–1 at 5 m depth, and followed the distribution patternof abundances. However, the distribution of Chl a differed considerablyfrom the distribution pattern shown by PPC. The algal assemblagein the sea ice, which could not be quantified due to high sedimentload, was dominated by diatom species, accompanied by dinoflagellates.Thus, already during the early stage of autumnal freeze-up,incorporation processes, selective enrichment and subsequentgrowth lead to differences between surface water and sea icealgal assemblages.     

Carbon:chlorophyll <Emphasis Type="Italic">a</Emphasis> ratio,assimilation numbers and turnover times of Lake Kinneret phytoplankton

Carbon to chlorophyll a (C:Chl) ratios, assimilation numbers (A.N.) and turnover times of natural populations of individual species and taxonomic groups were extracted from a long-term database of phytoplankton wet-weight biomass, chlorophyll a concentrations, and primary production in Lake Kinneret, Israel. From a database spanning more than a decade, we selected data for samples dominated by a single species or taxonomic group. The overall average of C:Chl was highest for cyanophytes and lowest for diatoms, while chlorophytes and dinoflagellates showed intermediate values. When converting chlorophyll a to algal cellular carbon this variability should be taken into account. The variability in C:Chl within each phylum and species (when data were available) was high and the variability at any particular sampling date tended to be greater than the temporal variability. The average chlorophyll a-normalized rate of photosynthetic activity of cyanophytes was higher and that of the dinoflagellates lower than that of other phyla. Turnover time of phytoplankton, calculated using primary productivity data at the depth of maximal photosynthetic rate, was longest in dinoflagellates and shortest in cyanophytes, with diatoms and chlorophytes showing intermediate values. The more extreme C:Chl and turnover times of dinoflagellates and cyanobacteria in comparison with chlorophytes and diatoms should be taken into consideration when employed in ecological modeling.     

Responses of phytoplankton to fish predation and nutrient loading in shallow lakes: a pan-European mesocosm experiment

1. The impacts of nutrients (phosphorus and nitrogen) and planktivorous fish on phytoplankton composition and biomass were studied in six shallow, macrophyte‐dominated lakes across Europe using mesocosm experiments. 2. Phytoplankton biomass was more influenced by nutrients than by densities of planktivorous fish. Nutrient addition resulted in increased algal biomass at all locations. In some experiments, a decrease was noted at the highest nutrient loadings, corresponding to added concentrations of 1 mg L^?1 P and 10 mg L^?1 N. 3. Chlorophyll a was a more precise parameter to quantify phytoplankton biomass than algal biovolume, with lower within‐treatment variability. 4. Higher densities of planktivorous fish shifted phytoplankton composition toward smaller algae (GALD < 50 μm). High nutrient loadings selected in favour of chlorophytes and cyanobacteria, while biovolumes of diatoms and dinophytes decreased. High temperatures also may increase the contribution of cyanobacteria to total phytoplankton biovolume in shallow lakes.     

Nutrient additions to wetlands in the Interlake region of Manitoba,Canada: effects of periodic additions throughout the growing season

The Interlake region of central Manitoba is characterized by numerous shallow, relatively unproductive wetlands. Typically, these wetlands are poorly utilized by breeding waterfowl in spite of generally reliable water conditions during spring and summer. Nutrient additions were made throughout the growing season to 18 PVC enclosures installed in a low productivity wetland near Lundar, Manitoba. Inorganic phosphorus (as H₃PO₄) and nitrogen (as NH₄NO₃) were added at bi-weekly intervals during the summer of 1988 at target rates of 0 and 0, 30 and 800, and 60 and 1600 µg 1^–1 (P and N respectively). Algal and invertebrate communities were monitored from mid-June to September, 1988. Phytoplankton, epiphytic periphyton and metaphyton communities demonstrated significant increases in biomass over the treatment period. No significant differences in epipelon community biomass were noted. An examination of several indicators of nutrient deficiency indicated that algal productivity was moderately to severely limited in all enclosures, with little or no mitigative effects noted due to nutrient addition treatment. No significant differences in numbers or biomass of total invertebrates or invertebrate functional groups attributed to fertilization were observed. Nutrient additions did increase community productivity, however the levels used in this study were insufficient to yield a sustained increase in primary or secondary productivity.     

Phytoplankton and physical-chemical features of Tavropos Reservoir,Greece

Phytoplankton biomass values in Tavropos Reservoir, ranging from 92 to 1071 mg m^–3, are within a range characteristic of oligotrophic waters. The seasonal sequence of biomass shows three annual peaks, differing from the monoacmic pattern seen in oligotrophic lakes. This sequence was profoundly affected by changes in water withdrawal and inflow rates. Diatoms, cryptophytes, chrysophytes and dinoflagellates, in that order, were the major constituents of the reservoir phytoplankton. The succession, from diatoms and chrysophytes in late winter-spring, to centric diatoms in late spring-summer and again to diatom-chrysophytes in late autumn was similar to that in oligotrophic lakes.     

Phytoplankton community responses to nutrient and iron enrichment under different nitrogen to phosphorus ratios in the northern Baltic Sea

The impact of nutrient enrichment on the phytoplankton community structure, and particularly cyanobacteria, was studied in a 3-week mesocosm experiment conducted in August 2001 in the Archipelago Sea, a part of the northern Baltic Sea. The factorial design experiment included daily additions of nitrogen (N) and phosphorus (P) at two mass ratios, 1N:1P and 7N:1P, respectively, additions of iron (Fe) and a synthetic chelator, ethylenediaminetetraacetic acid (EDTA). The floating enclosures (400 l) were sampled for analyses of phytoplankton biomass and community structure, phytoplankton primary production, chlorophyll a, nutrients, and hepatotoxins. Chlorophyll a concentration, phytoplankton biomass and primary production increased most in the 7N:1P treatment. The increase was mainly due to an abundant growth of chlorophytes (Dictyosphaerium subsolitarium, Kirchneriella spp., Monoraphidium contortum, and Oocystis spp.), pennate diatoms (especially Nitzschia spp.), dinophytes and the chroococcalean cyanobacterium Synechococcus sp. The nutrient enrichments had no effect on the total biomass of N₂-fixing cyanobacteria. Nevertheless, the biomass of Anabaena spp. was highest in the enrichments with a low N/P ratio. Chlorophyll a concentration and total phytoplankton biomass were not affected by Fe or EDTA, but Fe alone had a positive effect on the chlorophyte Kirchneriella sp. The N₂-fixing cyanobacteria Aphanizomenon sp. responded positively to Fe alone and to both Fe and EDTA added together. The hepatotoxin concentration increased during the experiment, but no clear responses to nutrient enrichments were found. Our study showed species-specific responses to nutrient enrichments among the N₂-fixing cyanobacteria. Although the total phytoplankton production was not Fe-limited; the availability of Fe clearly affected the phytoplankton community structure.     

RESPONSES OF THE PHYTOPLANKTON COMMUNITY GROWTH RATE TO NUTRIENT PULSES IN VARIABLE ESTUARINE ENVIRONMENTS

In estuaries, phytoplankton are exposed to rapidly changing conditions that may have profound effects on community structure and function. In these experiments, we evaluated the growth, productivity, and compositional responses of natural phytoplankton communities exposed to limiting nutrient additions and incubation conditions typical of estuarine habitats. Mesocosm bioassays were used to measure the short-term (2-day) growth rate, primary productivity, and group-specific biomass responses of the phytoplankton community in the Neuse River Estuary, North Carolina. A three-factor (mixing, sediment addition, and nutrient addition) experimental design was applied using 55-L mesocosm tanks. Growth rates were determined using the ¹⁴C photopigment radiolabeling method, and the abundance of algal groups was based on quantification of chemosystematic photopigments by HPLC. For Neuse River Estuary phytoplankton communities, stratified (nonmixed), turbid, and low-nitrate conditions favored increases in cryptomonad biomass. Mixed, turbid, high-nitrate conditions were favorable for increased primary productivity and chlorophytes, diatoms, and cyanobacteria. The highest community growth rates occurred under calm, high-nitrate conditions. This approach provided an assessment of the community-level phytoplankton responses and insights into the mechanisms driving blooms and bloom species in estuarine waters. The ability to rapidly alter growth rates to capitalize on conditions conducive for growth may play an important role in the timing, extent, and species involved with blooms in estuarine waters. Adaptive growth rate responses of individual species, as well as the community as a whole, further illustrate the sensitivity of estuarine ecosystems to excessive N inputs.     

Pigment-Based Chemotaxonomy - A Quick Alternative to Determine Algal Assemblages in Large Shallow Eutrophic Lake?

Pigment-based chemotaxonomy and CHEMTAX software have proven to be a valuable phytoplankton monitoring tool in marine environments, but are yet underdeveloped to determine algal assemblages in freshwater ecosystems. The main objectives of this study were (1) to compare the results of direct microscopy and CHEMTAX in describing phytoplankton community composition dynamics in a large, shallow and eutrophic lake; (2) to analyze the efficiency of the pigment-based method to detect changes in phytoplankton seasonal dynamics and during rapid bloom periods; (3) to assess the suitability of specific marker pigments and available marker pigment:chlorophyll a ratios to follow seasonal changes in eutrophic freshwater environment. A 5-year (2009-2013) parallel phytoplankton assessment by direct microscopy and by CHEMTAX was conducted using published marker pigment:chlorophyll a ratios. Despite displaying some differences from microscopy results, the pigment-based method successfully described the overall pattern of phytoplankton community dynamics during seasonal cycle in a eutrophic lake. Good agreement between the methods was achieved for most phytoplankton groups - cyanobacteria, chlorophytes, diatoms and cryptophytes. The agreement was poor in case of chrysophytes and dinoflagellates. Our study shows clearly that published marker pigment:chlorophyll a ratios can be used to describe algal class abundances, but they need to be calibrated for specific freshwater environment. Broader use of this method would enable to expand monitoring networks and increase measurement frequencies of freshwater ecosystems to meet the goals of the Water Framework Directive.     

The effect of seasonal variation in selective feeding by zebra mussels (Dreissena polymorpha) on phytoplankton community composition

1. To investigate the impact of zebra mussels ( Dreissena polymorpha ) on phytoplankton community composition, temporal variability in selective feeding by the mussels was determined from April to November 2005 in a natural lake using Delayed Fluorescence (DF) excitation spectroscopy.
2. Selective grazing by zebra mussels varied in relation to seasonal phytoplankton dynamics; mussels showed a consistent preference for cryptophytes and avoidance of chlorophytes and cyanobacteria. Diatoms, chrysophytes and dinoflagellates responded differentially to zebra mussel grazing depending on their size. Analysis of excreted products of the zebra mussels revealed that in addition to chlorophytes and cyanobacteria, phytoplankton >50  μ m and very small phytoplankton (≤7  μ m) were largely expelled in pseudofaeces.
3. The zebra mussel is a selective filter-feeder that alters its feeding behaviour in relation to phytoplankton composition to capture and ingest high quality phytoplankton, especially when phytoplankton occur in preferred size ranges. Flexibility of zebra mussel feeding behaviour and variation in susceptibility among phytoplankton groups to mussel ingestion indicate that invading zebra mussels could alter phytoplankton community composition of lakes and have important ecosystem consequences.     

Long-term trends and regional differences of phytoplankton in large Finnish lakes

Major nutrients (N and P) and phytoplankton from 19 large lakes from southern (61°) to northern (69°) Finland were analyzed to detect long-term trends and regional differences. The data sets from June, July and August cover the period from the early 1980s to the present. Altogether >700 phytoplankton and >4000 N, P and Chl a results were used for the study. In 40% of the lakes, the total phosphorus (TP) concentration decreased significantly and in >25% of the lakes a significant reduction was found in the total nitrogen (TN) concentration. At the same time, the phytoplankton biomass declined only in 15% of the lakes and the long-term trends in chlorophyll a more often increased than decreased. A clear gradient from southern to northern Finland and western to eastern Finland was found in the phytoplankton biomass. During the study period, the biomasses of cyanobacteria and centrales (diatoms) decreased whilst there was an increase in the biomass of pennales (diatoms) in one-third of the lakes. The proportion of chlorophytes in the total biomass also increased in >20% of the study lakes. In southern and western Finland, the total biomass and the contribution of cyanobacteria were higher. Centrales made a higher contribution to the total biomass in the north. Pennales and chlorophytes were less abundant and chrysophytes more abundant in the east. Differences in the community composition reflected the gradients in the total nutrients, and particularly in TP. The observations support the assumed role of phosphorus as the key limiting nutrient in large Finnish lakes irrespective of lake′s location. The N:P ratio proved to be a poor predictor of cyanobacteria occurrence in the study lakes.     

太平洋中西部海域浮游植物营养盐的潜在限制

2009年8月至9月期间在太平洋西部N1站和中部N2站进行现场营养盐加富培养实验。结果显示:N1站,浮游植物生物量对N或者P添加都有较强的响应,其中N+P+Si组和N+P组浮游植物长势迅速,叶绿素a从初始的0.03μg/L分别达到2.12μg/L和1.83μg/L,同时P先于N和Si之前被耗尽;说明N1站为N、P共同限制,P是首要限制因子。而N2站,浮游植物生物量仅对N、P共同添加有明显响应,N先于P和Si被浮游植物消耗殆尽。利用培养过程中营养盐比值变化推断,N1站浮游植物以低于Redfield ratio(16N∶1P)吸收N和P;而N2站浮游植物以高于Redfield ratio(16N∶1P)吸收N和P。这可能解释了太平洋西部的寡营养盐海域为潜在P限制,而在太平洋中部海域则为潜在N限制。     

Community composition has greater impact on the functioning of marine phytoplankton communities than ocean acidification

Ecosystem functioning is simultaneously affected by changes in community composition and environmental change such as increasing atmospheric carbon dioxide (CO₂) and subsequent ocean acidification. However, it largely remains uncertain how the effects of these factors compare to each other. Addressing this question, we experimentally tested the hypothesis that initial community composition and elevated CO₂ are equally important to the regulation of phytoplankton biomass. We full‐factorially exposed three compositionally different marine phytoplankton communities to two different CO₂ levels and examined the effects and relative importance (ω²) of the two factors and their interaction on phytoplankton biomass at bloom peak. The results showed that initial community composition had a significantly greater impact than elevated CO₂ on phytoplankton biomass, which varied largely among communities. We suggest that the different initial ratios between cyanobacteria, diatoms, and dinoflagellates might be the key for the varying competitive and thus functional outcome among communities. Furthermore, the results showed that depending on initial community composition elevated CO₂ selected for larger sized diatoms, which led to increased total phytoplankton biomass. This study highlights the relevance of initial community composition, which strongly drives the functional outcome, when assessing impacts of climate change on ecosystem functioning. In particular, the increase in phytoplankton biomass driven by the gain of larger sized diatoms in response to elevated CO₂ potentially has strong implications for nutrient cycling and carbon export in future oceans.     

A Compositional and Structural Analysis of Phytoplankton in Lake Khurpatal, (U.P.), India

The phytoplankton community in Lake Khurpatal (Kumaun Himalya), India, was analyzed in relation to physico-chemical variables. Phytoplankton biomass ranged from 2.7 to 20.0 g m⁻³ in the euphotic zone of the lake. Dinoflagellates monopolized the algal community with a mean annual contribution of 94.5 % to the total phytoplankton biomass. The community coefficient used to quantify the seasonal changes in algal population reflected the fact that successional episodes were not very significant, the percentage similarity among the consecutive algal samples ranging from 47.8 to 89.9 %. The phytoplankton community was characterized by low species richness, low equitability and species diversity. Various ecological characteristics of the community are discussed and the phytoplankton biomass is also compared with that of some tropical and temperate lakes.     

Phytoplankton Assemblages and Their Dominant Pigments in the Nervion River Estuary

In the Nervion River estuary surface samples were taken from March to September 2003 at six sites covering most of the salinity range with the aim to know the biomass and taxonomic composition of phytoplankton assemblages in the different segments. Nine groups of algae including cyanobacteria, diatoms, dinoflagellates, chlorophytes, prasinophytes, euglenophytes, chrysophytes, haptophytes, raphidophytes and cryptophytes were identified by means of a combination of pigment analysis by high-performance liquid chromatography (HPLC) and microscopic observations of live and preserved cells. Diatoms, chlorophytes and cryptophytes were the most abundant algae in terms of cells number, whereas fucoxanthin, peridinin, chlorophyll b (Chl b) and alloxanthin were the most abundant auxiliary pigments. Based on multiple regression analysis, in the outer estuary (stations 0, 1, 2 and 3) about 93% of the chlorophyll a (Chl a) could be explained by algae containing fucoxanthin and by algae containing Chl b, whereas in the rest of the estuary most of the Chl a (about 98%) was accounted for by fucoxanthin, Chl b and alloxanthin containing algae. The study period coincided with that of most active phytoplankton growth in the estuary and fucoxanthin was by far the dominant among those signature pigments. Several diatoms, chrysophytes, haptophytes and raphydophytes were responsible for fucoxanthin among identified species. Besides, dinoflagellates with a pigment pattern corresponding to chrysophytes and type 4 haptophytes were identified among fucoxanthin-bearing algae. Cryptophytes were the most abundant species among those containing alloxanthin. The maximum of Chl b registered at the seaward end in April coincided with a bloom of the prasinophytes Cymbomonas tetramitiformis, whereas the Chl b maxima in late spring and summer were accounted for by prasinophytes in the middle and outer estuary and by several species of chlorophytes in the middle and inner estuary. Other Chl b containing algae were euglenophytes and the dinoflagellate Peridinium chlorophorum. Dinoflagellates constituted generally a minor component of the phytoplankton.     

典型草原建群种羊草对氮磷添加的生理生态响应

由于人类活动和气候变化的共同作用, 大气氮(N)沉降日益加剧, 使得陆地生态系统中的可利用性N显著增加, 生态系统更易受其他元素如磷(P)的限制。然而, 目前关于N、P养分添加对草原生态系统不同组织水平的影响研究较少, 相关机制尚不清楚。该文以内蒙古典型羊草(Leymus chinensis)草原为研究对象, 通过连续两年(2011-2012年)的N和P养分添加实验, 研究建群种羊草的生理生态性状、种群生物量和群落初级生产力对N、P添加的响应及其适应机制。结果表明: 羊草草原不同组织水平对N、P添加的响应不同。群落水平上, 地上净初级生产力在不同降水年份均受N和P元素的共同限制, N、P共同添加显著提高了地上净初级生产力; 物种水平上, N、P添加对羊草种群生物量和密度, 以及相对生物量均没有显著影响, 表明羊草能够维持种群的相对稳定; 个体水平上, 在正常降水年份(2011年), 羊草生长主要受N素限制, 而在湿润年份(2012年), 降水增加使得羊草生长没有受到明显的养分限制。羊草通过增加比叶面积、叶片大小和叶片N含量, 提高整体光合能力, 以促进个体生长。总之, 内蒙古典型草原群落净初级生产力受N、P元素共同限制, 作为建群种的羊草, 其对N、P添加的响应因组织水平而异, 也受年际间降水变化的影响。     

Nutrient limitation of phytoplankton communities in Subarctic lakes and ponds in Wapusk National Park, Canada

We determined the limiting nutrient of phytoplankton in 21 lakes and ponds in Wapusk National Park, Canada, using nutrient enrichment bioassays to assess the response of natural phytoplankton communities to nitrogen and phosphorus additions. The goal was to determine whether these Subarctic lakes and ponds were nutrient (N or P) limited, and to improve the ability to predict future impacts of increased nutrient loading associated with climate change. We found that 38% of lakes were not limited by nitrogen or phosphorus, 26% were co-limited by N and P, 26% were P-limited and 13% were N-limited. TN/TP, DIN/TP and NO₃ ⁻/TP ratios from each lake were compared to the Redfield ratio to predict the limiting nutrient; however, these predictors only agreed with 29% of the bioassay results, suggesting that nutrient ratios do not provide a true measure of nutrient limitation within this region. The N-limited lakes had significantly different phytoplankton community composition with more chrysophytes and Anabaena sp. compared to all other lakes. N and P limitation of phytoplankton communities within Wapusk National Park lakes and ponds suggests that increased phytoplankton biomass may result in response to increased nutrient loading associated with environmental change.